This invention relates to a magnetic recording medium, and more particularly, to an improved magnetic recording medium having minimized head clogging and dropouts as well as excellent moving stability and runnability.
At present, magnetic recording media are widely used in a variety of applications including audio, video, computer, and other utilities in the form of magnetic tape, disc, etc. As the quantity of information required to record in magnetic recording media is increasing, a great improvement in recording density is increasingly demanded for such magnetic recording media.
Non-binder type magnetic recording media, that is, ferromagnetic thin film-bearing magnetic recording media are easier to fabricate in thinner form and have more saturation magnetization than coating type media. The non-binder type media, however, are rather insufficient in corrosion, impact, and frictional strength so that they might be subject to friction and even breakage due to high speed relative motion in contact with a magnetic head during recording, reproducing and erasing operations of magnetic signals. More particularly, since the ferromagnetic thin film is formed on a substrate by any desired processes such as electrodeposition, electroless deposition, sputtering, vacuum deposition or evaporation, and ion plating, the resulting magnetic medium is free of a binder so that the magnetic recording layer can be abraded away or broken by friction during sliding contact with a magnetic head.
In addition, the ferromagnetic thin film-bearing magnetic recording media are liable to corrosion at their surface. A progress of corrosion adversely affects physical properties such as head contact and abrasion resistance as well as electromagnetic properties.
To overcome these drawbacks, Japanese Patent Publication No. 39-25246 proposed to apply a lubricant to the surface of a ferromagnetic metal thin film. Simple coating of lubricant fails to provide continuous lubrication because it is readily removed off by a magnetic head and other rigid members. The application of lubricant is, of course, not expected of rust prevention and durability.
To continuously provide lubrication onto the magnetic recording layer, it is proposed in Japanese Patent Publication No. 57-29769 to form a lubricating or backcoat layer of an organic binder containing a liquid or semi-solid lubricant on that surface of a substrate remote from the magnetic recording layer. The lubricant oozing out of the back surface of the medium is transferred to the magnetic recording layer when the medium is wound in a roll form so that the lubricant is continuously fed to the surface of the magnetic recording layer. There are thus obtained excellent effects in the durability (abrasion, wear or separation resistance) and coefficient of dynamic friction of the magnetic recording layer. This type of magnetic recording medium utilizing continuous feed of lubricant from the backcoat layer to the magnetic thin film without a topcoat layer has a relatively high level of friction between the magnetic thin film and a magnetic head so that the movement of the medium is not satisfactorily smooth and the prevention of corrosion or rust of the magnetic thin film is still insufficient.
In the coating type of magnetic recording medium, topcoat layers are formed for the protection of the magnetic recording layer. Prior art topcoat layers are readily peeled off.
To obviate these drawbacks, the inventors previously proposed in Japanese Patent Application Nos. 59-15258, 59-15259, and 59-77583 a magnetic recording medium comprising a magnetic recording layer formed on a non-magnetic substrate wherein a topcoat layer of a specific composition is formed on the magnetic recording layer to thereby improve the dynamic properties and durability of the medium. We have found that the previously proposed topcoat layers are relatively readily peeled off because they are free of fine pigment particulates and thus cannot provide a cleaning effect to a head.
The present invention is based on our discovery that when the topcoat layer is observed under an electron microscope to contain submicron particulates of a pigment having a particle size of less than 200 .ANG. in a linear distribution density or population of 10 to 1,000 per linear 100 .mu.m and to have a surface roughness R20 of not more than 400 .ANG., there is obtained an improved magnetic recording medium which has eliminated the above-mentioned drawbacks while exhibiting superior properties including minimized head clogging, level down, dropout, friction, and abrasion as well as stable electromagnetic characteristics.